Pick-and-Remove System and Method for Emissive Display Repair

ABSTRACT

A system and method are provided for repairing an emissive display. Following assembly, the emissive substrate is inspected to determine defective array sites, and defect items are removed using a pick-and-remove process. In one aspect, the emissive substrate includes an array of wells, with emissive elements located in the wells, but not electrically connected to the emissive substrate. If the emissive elements are light emitting diodes (LEDs), then the emissive substrate is exposed to ultraviolet illumination to photoexcite the array of LED, so that LED illumination can be measured to determine defective array sites. The defect items may be determined to be misaligned, mis-located, or non-functional emissive elements, or debris. Subsequent to determining these defect items, the robotic pick-and-remove process is used to remove them. The pick-and-remove process can also be repurposed to populate empty wells with replacement emissive elements.

RELATED APPLICATIONS

This application is a Continuation-in-part of an application entitledEMISSIVE DISPLAY WITH PRINTED LIGHT MODIFICATION STRUCTURES, invented byUlmer et al., Ser. No. 15/413,053, filed Jan. 23, 2017, attorney docketNo. eLux_00105;

which is a Continuation-in-part of an application entitled SYSTEM ANDMETHOD FOR THE FLUIDIC ASSEMBLY OF EMISSIVE DISPLAYS, invented by Sasakiet al, Ser. No. 15/412,731, filed Jan. 23, 2017, attorney docket No.eLux_00101;

which is application is a Continuation-in-part of an applicationentitled EMISSIVE DISPLAY WITH LIGHT MANAGEMENT SYSTEM, invented byUlmer et al, Ser. No. 15/410,195, filed Jan. 19, 2017, attorney docketNo. eLux_00102;

which is a Continuation-in-part of an application entitled DISPLAY WITHSURFACE MOUNT EMISSIVE ELEMENTS, invented by Schuele et al., filed Jan.19, 2017, Ser. No. 15/410,001, attorney docket No. eLux_00100;

which is a Continuation-in-part of application Ser. No. 14/749,569,invented by Sasaki et al., entitled LIGHT EMITTING DEVICE AND FLUIDICMANUFACTURE THEREOF, filed on Jun. 24, 2015;

Ser. No. 15/410,001 is also a Continuation-in-part of application Ser.No. 15/221,571, invented by Crowder et al., entitled SUBSTRATE WITHTOPOLOGICAL FEATURES FOR STEERING FLUIDIC ASSEMBLY LCD DISKS, filed onJul. 27, 2016;

Ser. No. 15/410,001 is also a Continuation-in-part of application Ser.No. 15/197,226, invented by Kurt Ulmer, entitled LAMINATED PRINTED COLORCONVERSION PHOSPHOR SHEETS, filed on Jun. 29, 2016;

Ser. No. 15/410,001 is also a Continuation-in-part of application Ser.No. 15/190,813, invented by Schuele et al., entitled DIODES OFFERINGASYMMETRIC STABILITY DURING FLUIDIC ASSEMBLY, filed on Jun. 23, 2016;

Ser. No. 15/410,001 is also a Continuation-in-part of application Ser.No. 15/158,556, invented by Zhan et al., entitled FORMATION ANDSTRUCTURE OF POST ENHANCED DIODES FOR ORIENTATION CONTROL, filed on May18, 2016;

Ser. No. 15/410,001 is also a Continuation-in-part of application Ser.No. 15/266,796, invented by Heine et al., entitled SUBSTRATE FEATURESFOR ENHANCED FLUIDIC ASSEMBLY OF ELECTRONIC DEVICES, filed on Sep. 15,2016;

Ser. No. 15/410,001 is also a Continuation-in-part of application Ser.No. 14/680,618, invented by Zhan et al., entitled FLUIDIC ASSEMBLYTOP-CONTACT DISK, filed on Apr. 7, 2015:

which is a Continuation-in-part of application Ser. No. 14/530,230,invented by Zhan et al., entitled COUNTERBORE POCKET STRUCTURE FORFLUIDIC ASSEMBLY, filed on Oct. 31, 2014.

All the above-listed applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to fluidically assembled emissivedisplays and, more particularly, to a system and method for the repairof emissive displays.

2. Description of the Related Art

The current competing technologies for large area display are liquidcrystal display (LCD), organic light emitting device (OLED) display, andmore recently, inorganic LED display. The weaknesses of LCD are: 1) lowefficiency where only about 5% of the light generated by the backlightis seen as an image by the user, and 2) low dynamic range because the LCmaterial cannot completely block light to produce a black pixel. Theweaknesses of OLED displays are poor reliability and low efficiency (˜5%quantum efficiency (QE)) of the blue OLED material. The use of inorganicmicro-LEDs (uLEDs) in a display would provide a very high efficiencybecause the display would not use color filters and polarizers to absorblight. As used herein, a uLED is an LED with a diameter orcross-sectional area of 100 microns or less. The inorganic uLED displaywould have very high contrast because black pixels are set to emit nolight. For an inorganic uLED display, blue gallium nitride (GaN) LEDswould be 35-40% efficient, with a reliability of over 50,000 hours, ashas been established in general lighting. Sony has developed a passivematrix of uLEDs arranged in a display array using a pick and placesystem. However, since large displays require millions of LEDs, displaysmade by this process are time and cost prohibitive compared to othertechnologies.

The fluidic transfer of microfabricated electronic devices,optoelectronic devices, and sub-systems from a donor substrate/wafer toa large area and/or unconventional substrate provides a new opportunityto extend the application range of electronic and optoelectronicdevices. For example, display pixel size LED micro structures, such asrods, fins or disks, can be first fabricated on small size wafers andthen be transferred to large panel glass substrate to make a directemitting display requiring no backlighting.

As in any emissive display fabrication process, opportunities arisewhere the LEDs becomes mis-located or damaged. Considering the fact thata large area display may be comprised of millions of LEDs, the detectionand replacement of defective LEDs can be an extensive chore. For thestamp-transfer assembly of arrayed microcomponents, one process (U.S.Pat. No. 7,723,764) electrically tests fully integrated arrays toidentify defective components followed by excising the defectivecomponent's driving lines and rerouting them to a replacement componentthat is mounted atop the defective one.

More generally, substrate-wide processing steps are often used tomitigate the negative effects of missing, mis-located, or brokencomponents by insulating electrical contacts, covering the defect, andexcising electrical contacts. U.S. Pat. No. 9,252,375 describes theinspection and selective passivation of missing or defective arrayedcomponents, as well as excision of driver circuit leads to such defects.Most often, emphasis is placed on the production of defect-free arrays.For arrays on the scale of millions of components, however, even verylow defect rates can result in an unusable product.

Likewise, defects can occur when using fluidic assembly processes tofabricate large area displays. Therefore, it would be desirable todevelop the capability of repairing the low-rate of defects that resultfrom fluidic self-assembly. More explicitly, it would be advantageous ifa systematic approach existed for identifying the locations of emptywells or broken emissive elements and subsequent correction, followed byselective removal of all residual unaligned components within the devicearea.

SUMMARY OF THE INVENTION

Disclosed herein is a process by which a relatively low number ofdefects resulting from fluidic self-assembly of an emissive display canbe systematically identified and repaired. The inspection of eachalignment site is necessary for verifying the occupancy of an intact andcorrectly oriented component. While inspection can be done withmicroscopy and digital image processing approaches, which are standardin industrial electronics fabrication, in one aspect emissions areinduced in assembled arrays to additionally identify correctly locatedand aligned components that appear whole but are non-functional.Incorrectly located and misoriented devices are also considerednon-functional.

The output of the initial inspection test determines if a site isoccupied by a functional component, occupied by a nonfunctional orfractured component, or is unoccupied. With the repair operations, thefirst step is removal of the nonfunctional or fractured components fromalignment sites. The second step in repair is to fill the unoccupiedsites with functional components. The source of these components may bethe unaligned components from the field or a reservoir of fresh emissiveelements sufficiently spaced for individual pickup. Both removal andreplacement steps may be accomplished by single-componentpick-and-remove subsystem. Alternatively, replacement may be achieved byone or more repeated fluidic assembly steps.

The third step in repair is the removal of residual uncapturedcomponents. In fluidic assembly, deterministic control over individualcomponent trajectories is not always possible, and after assembly,components may reside between alignment sites on the receiving substratesurface. For low fill-factor arrays, capture sites represent a smallpercentage of the total array area and identifying the location ofindividual mis-located components is both expensive and unnecessary.Rather, the residual mis-located components are removed in a singlelarge-scale step that selects for mis-located components over correctlylocated components. The success of the repair steps is verified with afinal inspection prior to further integration of components to thereceiving substrate. If this inspection reveals persisting defects inthe array, the repair process may be iterated.

Accordingly, a method is provided for repairing an emissive display. Themethod provides an emissive substrate including an array of positionedemissive elements. Following assembly, the emissive substrate isinspected to determine defective array sites, and defect items areremoved from the emissive substrate. In one aspect, the emissivesubstrate includes an array of wells, with emissive elements located inthe wells, but not electrically connected to the emissive substrate.

In another aspect, the emissive elements are light emitting diodes(LEDs). Then, inspecting the emissive substrate includes irradiating theemissive substrate with ultraviolet (UV) illumination, photoexciting thearray of LED, and using an optically filtered inspection to distinguishdefective array sites from those with functional LEDs. The defect itemsmay be determined to be missing emissive elements, misaligned,mis-located, or non-functional emissive elements, or debris (e.g.,broken emissive element parts). Subsequent to determining misaligned,mis-located, non-functional emissive elements, or debris, a roboticpick-and-remove process is used to remove the defect item. The roboticpick-and-remove process may use an electrostatic, mechanical, oradhesive holding mechanism, as explained in more detail below.

The positioning of replacement emissive elements in the defective arraysites may be accomplished using a fluidic assembly or a repurposedpick-and-remove process. Subsequent to positioning replacement emissiveelements in any empty wells, the emissive substrate is reinspected todetermine defective array sites. If reinspection is passed, the emissivesubstrate is annealed so as to electrically connect the emissiveelements to the emissive substrate.

Additional details of the emissive substrate repair process and anemissive substrate repair system are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic block diagrams of an emissive displayrepair system.

FIGS. 2A and 2B are diagrams depicting an electrostatic pick-and-removedevice.

FIGS. 3A through 3F depict an exemplary mechanical pick-and-removedevice.

FIGS. 4A through 4C depict an adhesive pick-and-remove device.

FIGS. 5A through 5C depict an exemplary emissive element replacementprocess.

FIG. 6 is a high-level repair process flowchart.

FIGS. 7A through 7D are plan views of an exemplary emissive substratefollowing a fluidic assembly process.

FIGS. 8A and 8B are a flowchart illustrating a method for repairing anemissive substrate.

DETAILED DESCRIPTION

FIGS. 1A through 1C are schematic block diagrams of an emissive displayrepair system. The system 100 comprises an inspection subsystem 102 forinspecting an emissive substrate 104 with an array of wells 106. Asshown, emissive elements 108 are located in the wells 106, but notelectrically connected to the emissive substrate 104. Alternatively butnot shown, the emissive elements may be located at predetermined arraysites on a planar emissive substrate top surface. Typically, theemissive elements 108 are deposited in the wells 106 via a fluidicassembly process, as explained in the parent applications listed in theRELATED APPLICATIONS Section, above. However, the wells may also bepopulated using a conventional robotic pick-and-place device. Thepurpose of the inspection subsystem is to determine defective arraysites. Defective array sites 110 (circled in phantom) are shown.

Advantageously, the defective array sites are determined without theneed for electrical (i.e., soldered) connections to the substrate, sothat defective emissive elements 108 can be more easily replaced. System100 also comprises a pick-and-remove subsystem 112 for removing defectitems 113 from the emissive substrate 104.

In one aspect, the emissive elements 108 are light emitting diodes(LEDs). In this case, the inspection subsystem 102 comprises anilluminator 116 for irradiating the emissive substrate 104 (orindividual LEDs 108) with ultraviolet (UV) spectrum light andphotoexciting the LEDs. A dual-mode image sensor 118 identifies thepresence of LEDs 108 through visual contrast and edge detection in onemode, while in another mode uses wavelength specific filtration toidentify functional LEDs 108 by detecting the desired photoluminescencecaused by photogenerated carriers.

For example, a UV laser, a UV LED, a xenon arc lamp, a mercury arc lamp,or a xenon mercury arc lamp may be used as the UV emission unit 110. Ifthe LEDs 108 have leakage current, light emission caused by thephotoluminescence effect in the semiconductor layers occurs dominantly,as the recombination of excited electrons is non-emissive. If the LEDs108 are not defective, the photoluminescence effect occurs in both theactive layer and the semiconductor layers. In this case, light emissiondue to the photoluminescence effect in the active layer becomesdominant, and thus the light being generated has a different color thanthe defective LED. Hence, light having a predetermined wavelength isgenerated and thus allows the determination of whether an LED 108 isdefective.

The image sensor 118 captures the wavelengths of light generated bydefective, non-defective, and missing LEDs 108, and compares themeasurements to a predetermined standard. In other aspects, inspectionimaging can use spectroscopy instead of wavelength selective filters tomore precisely quantify the photoluminescence of UV-excited LEDs. Theinspection of LEDs can include a non-binary brightness assessment with auniformity criterion to determine removal thresholds, and the inspectionof LEDs can investigate red-blue-green (RGB) color balance per pixel forlater correction.

A spectroscope gathers all the emitted light and records thedistribution. This measurement generally does not include position data,as would be the case with a charge-coupled device (CCD) or CMOS sensor,so the position data must come from recording the xy (horizontal)position of the inspection head. This means that only one LED can beinspected at a time. In contrast, a bandgap filtered camera can inspecta larger field of view (but with less quantitation of wavelength).

In the case of gallium nitride (GaN) LEDs, the predominant wavelength isin the blue or green color spectrum, depending the LED doping. In thecase of AlGaInP LEDs, the predominant wavelength is in the red colorspectrum. The filtered image sensor compares detected photoluminescenceto a predetermined map cross-referencing position on the substrate toexpected wavelengths. Wavelengths of light either missing or notmatching the map of expected colors determine an array site to bedefective. The inspection is done prior to the application of any colorfilter or color modification layers. Thus, depending the display design,the emissive substrate 102 may be fabricated with one type (one color)of LED, two types (two colors) of LEDs, or three types (three colors) ofLEDs. In one aspect, the image sensor may be replaced with aspectroscope enabling quantitative wavelength measurements through anecessarily smaller field of view. In another aspect, the filtered imagesensor 118 compares measured desired wavelength light intensity to apredetermined standard to determine if an LED is defective. In summarythen, the inspection subsystem 102 determines defect items such asmissing emissive elements (wells not populated by an emissive element),misaligned emissive elements (wells populated with an “upside-down”emissive element), mis-located (LEDs not located in a well),non-functional emissive elements, and debris (e.g., broken emissiveelements, debris resulting from the fabrication of the emissivesubstrate, or solid objects in the fluidic assembly fluid). In the caseof misaligned, mis-located, or non-functional emissive elements ordebris, the pick-and-remove subsystem 112 uses a robotic pick-and-removedevice to remove these defect items 113, as explained in more detailbelow, the robotic pick-and-place devices uses one of the followingholding mechanisms: electrostatic, mechanical, and adhesive. As would beunderstood in the art, the pick-and-remove device includes aconventional optics/camera subsystem and/or a system for preciselymeasuring a destination (defect item) with respect to a known referencesuch as a substrate edge or corner.

FIGS. 2A and 2B are diagrams depicting an electrostatic pick-and-removedevice. Microcomponent emissive elements are held to a transfer head bycreating an electrostatic charge on the transfer head that inducescharge separation and attraction in the microcomponent. Release isachieved by removing the charge separation in the head. Theelectrostatic pick-and-remove device 200 comprises a transfer head 202capable of creating an electrostatic charge to attract a defect item 204to the transfer head (FIG. 2A). Reference designator 206 represents anattraction force due to the electrostatic charge. The electrostaticcharge on the transfer head 202 can be dissipated to release the defectitem 204 (FIG. 2B). In one aspect, the transfer head is basically acapacitor concentrating charge on pick-up surface (protected by thindielectric). A positive charge, for example, on transfer head attractselectrons to the LED top surface, creating a small attractive force.This process is effective because the emissive element has very littlemass.

FIGS. 3A through 3F depict an exemplary mechanical pick-and-removedevice. The transfer pickup head may be a thin, heatable metal tipconnected to a stepper-controlled xyz (capable of movement in threedimensions) transfer head stage. The tip is coated with phase-changematerial and approaches the substrate surface at the location of thedefect item to be removed, followed by resistive heating of the tipinducing melting of the coating material such that it contacts anycomponents, fragments, or debris in the well or substrate top surface.The coating material then solidifies on cooling, removing any loosematerial from the site when the tip is removed. The material and coatingare removed in a bath or spray of solvent to dissolve the coatingmaterial and the tip is then recoated with fresh material by dipping ina liquid bath of the phase-change polymer. Alternatively, the transferhead 302 is disposable and thrown out with the attached defect item 310after use. A strong advantage of this approach is that the z-height(vertical) control near the defective array site may be relatively lowprecision as the phase change material will droop under gravity to makecontact, and removes a wider variety of particle sizes and shapes thanelectrostatic or elastomeric adhesion. The mechanical force is alsocapable of exceeding electrostatic or the elastomeric adhesive force.

Thus, the mechanical pick-and-remove device 300 comprises a thermaltransfer head 302 with a liquid phase polymer coating 304 overlying thethermal transfer head. The thermal transfer head 302 may be heated, asrepresented by voltage potential 306 (FIG. 3A), to convert a solid phasepolymer to a liquid phase 308 or to maintain a polymer in its liquidphase. Subsequent to contacting a defect item 310 (FIG. 3B), the defectitem becomes attached to the transfer head. As shown in FIGS. 3C and 3D,the transfer head 303 is cooled to convert the liquid phase polymer to asolid phase 312 attached to the defect item 310. In FIG. 3E the transferhead 302 is cleaned of the polymer with liquid 314 to remove the defectitem, and in FIG. 3F the transfer head 302 is recoated with a polymer304 from a liquid phase polymer bath 316.

FIGS. 4A through 4C depict an adhesive pick-and-remove device. In oneaspect, the defect item and the transfer head are natively adherent,with an overall binding strength scaling with interfacial area. Releaseis achieved by deflecting of the elastomeric surface which reducescontact area with the rigid defect item microcomponent, reducing theholding force between the transfer head and defect item. The adhesivepick-and-remove device 400 comprises a transfer head 402 with adeformable contact surface area 404. The contact surface area 404 isadhesive, either natively so, or coated with an adhesive layer (FIG.4A). Since the transfer head 402 does not directly contact the surfaceof the emissive substrate 104, the transfer head becomes adhesivelyattached a defect item 408 in response to expanding (deforming) thecontact surface area size, as shown in FIG. 4B. The defect item isreleased in response to bathing the transfer head in a solvent orincreasing the deformation of transfer head to reduce the amount ofsurface area contacting the defect item (not shown).

One variation of the adhesive approach is to coat the transfer head witha liquid, concomitant with the use of a substrate that does not retainany liquid after contact. One example is a hydrophobic substrate surfaceand a polar liquid that holds the to-be-removed defect item throughsurface tension after contact.

Returning to FIG. 1C, the repair system 100 may further comprise areplacement subsystem. Empty wells 106 in the emissive substrate can bepopulated using a fluidic assembly process, or as shown, a repurposedpick-and-remove subsystem 120.

FIGS. 4A through 4C may also be interpreted as steps in the removal of adefective emissive element from a substrate well using a deformableelastomeric transfer head. The repair of fluidically assembled arrayscan be broadly reduced to two fundamental operations: the removal ofemissive elements from the substrate, and the addition of replacementemissive elements into the emissive substrate wells. Removal of brokenor nonfunctional components 408 from the wells 106 requires thepick-and-remove transfer head 402 to overlap the alignment site (e.g.,well) before intimate approach and pickup. For removal, the relativeposition of the transfer head 402 and emissive element component 408 isnot critical and can generally be achieved without additional positionalfeedback in the form of cameras or linear encoders. After pickup, thepart is translated away from the assembly area and discarded while thehead is reset to pick up and remove the next component. For emissiveelements recessed in wells, an adherent elastomeric pick-up head may bedeformed to contact the component. For electrostatic heads, an increasedfield may be necessary to overcome increased distance and the squaredlaw drop off in grip strength. Phase-change material-coated pickup headsare also efficacious at removal of broken components and debris fromwells.

After defective and broken components are removed from wells, theirprevious locations and the initially empty sites are targeted forassembly of new replacement components. This may be done in a similarmethod to the initial assembly—via fluidic self-assembly, and theinspection/removal steps are repeated until the array achieves desiredfunctional yield. Alternatively, the pick-and-remove xyz transfer headthat was used for removal may be repurposed and used to place newcomponents. The addition process requires significantly higher precisionin placement than removal, so after new components are picked up from astaging area, the transfer head passes over an up-looking camera thatcorrects the relative position between the component center and the headcenter. For radially asymmetric components, angular orientation may alsobe corrected at this point. The part is then placed into the substratewells and translated to deposit the component in the recess. Forelectrostatic adhesion, component deposition may be achieved byde-energizing the electric field, but native stiction for microscalecomponents may necessitate mechanically-assisted detachment.

If the reliability of the pick-and-remove translation is insufficientfor the scale of the array and the components, a compliant head capableof deflecting, without damage to carried components, is located near thealignment site and lightly presses the component against the assemblysurface. The head then translates the contacted component in the regionof the recess such that the component is forced into the recess andmechanically retained as shown in FIGS. 5A through 5C. Due to the scaleand brittle nature of the components, the down-force is carefullycontrolled and monitored such as with a piezoelectric force gauge in thepick-and-remove head. In this manner, without perfect knowledge andcontrol over the relative positioning of the picked up component and thewell, assembly can still be achieved.

The final repair step is a large-area clean that uses differentialforcing to remove any out-of-place (mis-located) components from thesubstrate. In this case the wells comprise the alignment sites andin-field emissive element components are located on the substratesurface and not laterally confined by the wells. As such, an adherentsurface brought into close contact with the substrate surface exerts asignificantly stronger force on an out-of-place components than therecessed correctly located components. This attractive force may beprovided by coulombic, dielectrophoretic, or chemical adhesion. Anadditional approach leverages the lateral retention of the wells oncorrectly located components and provides a mechanical shear force onthe substrate surface to dislodge mis-located components. The shearforce may be provided by fluid flowing across the substrate or directforcing provided by a brush or solid surface. A tilted substrate andgravitational forcing may also be used to direct unretained componentsout of the assembly area into a collection trough. In this case, thesubstrate may be coupled to a directionally vibrating oscillator toreduce component stiction and the substrate may be covered in a carrierfluid to assist the transit of misaligned components.

The form of non-fluidic and non-gravitational final clean-off may be acylinder that transits over the surface to remove mis-locatedcomponents, a rigid sheet of dimensions comparable to the assemblysubstrate dimensions, a pliant sheet or brush of critical dimensionsgreater than the component's so as not to dislodge correctly locatedcomponents while exerting shear force on the substrate surface, or apliant natively adherent sheet such as, for example,polydimethylsiloxane (PDMS), which pulls mis-located components from thesubstrate when the sheet is peeled off.

In addition to assembly based on component retention in recessed wells,these approaches are also applicable to any alternate assembly schemeswhere correctly located components are held more tightly thanout-of-place components, and by using a driving force with magnitudebetween the two adhesion forces. Out-of-place components may then berecycled into ink for future fluidic assembly.

After repair, the substrate is again inspected and verified that allalignment sites are occupied by intact and functional components and noresidual out-of-place components remain on the substrate.

FIGS. 5A through 5C depict an exemplary emissive element replacementprocess. Using any of the above-described robotic pick-and-removedevices as repurposed to deposit emissive elements, a transfer head 500is shown with an attached replacement emissive element 502. The transferhead 500 positions the replacement emissive element 502 on the emissivesubstrate 104 top surface 504 at a location proximate to a well 106 tobe populated (FIG. 5A). The transfer head 500 translates the replacementemissive element 502 across top surface 504, as represented by arrow506, over a well 106 opening, forcing the replacement emissive elementinto the well (FIGS. 5B and 5C).

FIG. 6 is a high-level repair process flowchart. The system disclosedherein is a well suited for processes in which a relatively low numberof defects resulting from fluidic self-assembly can be systematicallyidentified and repaired. In Step 602 the emissive substrate is prepared,including a matrix of column and row lines needed to selectively enableindividual emissive elements, and optionally including active matrixdrive circuitry, as described in the parent applications listed in theRELATED APPLICATIONS Section, above. Step 602 also includes theformation of wells in the emissive substrate top surface. In Step 604 afluidic assembly process positions emissive elements in the emissivesubstrate wells. In Step 606 an initial inspection is preformed, usingthe inspection subsystem described in FIG. 1. Inspection of eachemissive element site is necessary for verifying the occupancy of anintact and correctly oriented component. While inspection can be donewith microscopy and digital image processing approaches which arestandard in industrial electronics fabrication, emission can also beinduced using UV radiation to additionally identify correctly locatedand aligned components that appear whole but are non-functional.Correctly located but misaligned devices are also considerednon-functional.

In Step 608 defective array sites are repaired. The output of theinitial inspection test (Step 606) is a trinary array corresponding toknown alignment sites and indicating if the site is: occupied by afunctional component, occupied by a nonfunctional component, debris, orunoccupied. Step 608 a removes nonfunctional components or debris fromalignment sites. The successful execution of this step effectivelycreates an array with wells in a binary condition, describing sites thatare either occupied by a functional component or empty.

Step 608 b fills the unoccupied sites with functional components. Thesource of these components may be the mis-located components from thefield (substrate surface) or a reservoir of fresh emissive elementssufficiently spaced for individual pickup by a pick-and-remove device.Thus, both Steps 608 a and 608 b may be accomplished by single-componentpick-and-remove operations. Alternatively, the unoccupied wells may befilled using a second fluidic assembly process.

In Step 608 c residual mis-located emissive elements are removed. Amis-located emissive element occupies a place on the emissive substrateoutside of a well or assigned position on the substrate surface. Influidic assembly (Step 602), deterministic control over individualcomponent trajectories is not always possible, and after assembly,mis-located components may reside between wells on the receivingsubstrate surface. For low fill-factor arrays, alignment sites representa small percentage of the total array area and identifying the locationof individual mis-located components is both expensive and unnecessary.Rather, the residual mis-located components may be removed in a singlelarge-scale step that selects for mis-located components over correctlylocated components. For example, a brush, wiper, gas, or liquid can beapplied to the emissive substrate top surface. Alternatively, if Step608 b used a fluidic assembly process, Steps 608 b and 608 c may becombined.

The success of these repair steps is verified with a final inspectionprior (Step 610), followed by the further integration of components tothe receiving substrate in Step 612. If this inspection revealspersisting defects in the array, the repair process is iteratedaccordingly.

Inspection of the rest of the substrate may be undertaken to assess theextent of residual emissive elements, but the most facile aspect ofmis-located component removal is a selective large-scale operation.Otherwise, this process limits initial inspection (Step 606) to wellsites and includes an examination of the full-substrate area in thefinal inspection (Step 610) before integration. Thus, two inspectionmethods are presented: large-area inspection and site-by-siteinspection. If the fluidically assembled components are micro-sized LEDs(uLEDs), having a diameter or cross-section of less than 100 microns,the fundamental mechanism behind both may be photoexcitation of the uLEDwith UV illumination and wavelength-selective measurement to identifythe presence and function of correctly located uLEDs. With sufficientlyefficient optics, large-area imaging can characterize uLED dispositionover the assembly substrate. If site-by-site inspection or imaging lowerthan full-area, the imaging system is either arrayed or transited overthe assembly substrate surface and the processed image data is used togenerate a matrix corresponding to the substrate alignment sites beingfunctional, unoccupied, mis-located, occupied by nonfunctionalcomponents, or detecting debris.

FIGS. 7A through 7D are plan views of an exemplary emissive substratefollowing a fluidic assembly process. As integration relies on wholecomponents for electrode contacts, broken but functional components arealso removed. FIG. 7A depicts the results of a visual inspection. Asshown, most of the wells 106 are occupied with an emissive element 108,but some wells are unoccupied. Assuming the emissive elements are LEDs,FIG. 7B depicts the result obtained in response to exposing the emissivesubstrate to UV radiation. Some sites 300, marked with an “x” areoccupied but do not respond with the expected intensity or wavelength,indicating the occupying LED is defective. FIG. 7C depicts wells 302where defective LEDs are to be removed, and FIG. 7D depicts wells 304 tobe repopulated with replacement LEDs.

In one aspect, the repair tool is a 3-axis pick-and-remove head capableof handling microcomponents for repair of fluidic assembly's primarydefect modes: missing components, misaligned components, and mis-locatedcomponents residual on the substrate surface, and broken componentsoccupying alignment sites. Industry standard pick-and-place operationsare conventionally performed with a pneumatic pressure-based holdingforce between head and component, which requires the vacuum port to besmaller than the component handling face. In the case ofmicrocomponents, the vacuum-based approach becomes less appropriate asthe micro-scale port diameter restricts gas flow, creating significantpneumatic resistance that slows operation. Additionally, such smallports become susceptible to clogging. At the micro-scale, alternatehandling approaches are desirable.

For use with the repair system described herein, the pick-and-removetransfer head contact face may be smaller than the minimum array pitch(between wells) and larger than the emissive element contact face, sothat it is capable of transferring single microcomponents. As notedabove, the method for holding components to the transfer head may beelectrostatic, mechanical, or adhesive. Alternatively, the transfer headmay incorporate a mechanical attachment such as microelectromechanicalmachine system (MEMS) tweezers, topographic retention features, orvacuum pulled through a microporous feature with pores significantlysmaller than component dimensions. For components without radialsymmetry, a 4-axis pick-and-place transfer head may be used.

FIGS. 8A and 8B are a flowchart illustrating a method for repairing anemissive substrate. Although the method is depicted as a sequence ofnumbered steps for clarity, the numbering does not necessarily dictatethe order of the steps. It should be understood that some of these stepsmay be skipped, performed in parallel, or performed without therequirement of maintaining a strict order of sequence. Generallyhowever, the method follows the numeric order of the depicted steps. Themethod starts at Step 800.

Step 802 provides an emissive substrate including an array of positionedemissive elements. Step 804 inspects the emissive substrate to determinedefective array sites. Step 806 uses a pick-and remove process to removedefect items from the emissive substrate defective array sites.Subsequent to populating empty wells with replacement emissive elementsin Step 808, Step 810 reinspects the emissive substrate to determinedefective array sites, and subsequent to passing reinspection, Step 812anneals the emissive substrate. In response to the annealing, Step 814electrically connects the emissive elements to the emissive substrate.

In one aspect, Step 802 provides an emissive substrate with an array ofwells, with emissive elements located in the wells, but not electricallyconnected to the emissive substrate. If the emissive elements are LEDs,inspecting the emissive substrate in Step 804 includes substeps. Step804 a irradiates the emissive substrate with UV illumination. Step 804 bphotoexcites the array of LED, and Step 804 c measures LED illuminationat predetermined wavelengths to determine defective array sites. Thedefect items may include misaligned emissive elements, mis-locatedemissive elements, non-functional emissive elements, or debris.Subsequent to determining the above-mentioned defect items, Step 806uses the robotic pick-and-remove process to remove emissive elementsfrom the defective array sites. The robotic pick-and-remove process usedmay use one of the following holding mechanisms: electrostatic,mechanical, or adhesive.

In the case of the electrostatic mechanism, Step 806 a creates anelectrostatic charge between a pick-and-remove transfer head and adefect item. Step 806 b attracts the defect item to the transfer head inresponse to the electrostatic charge, and Step 806 c either removes(dissipates) the electrostatic charge to release the defect item fromthe transfer head, or the step disposes of the transfer head withattached defect item.

In the case of the mechanical mechanism, Step 806 d coats apick-and-remove transfer head with a liquid polymer. Subsequent tocontacting a defective emissive element with the transfer head, Step 806e permits the transfer head to cool, Step 806 f converts the polymer toa solid phase attached to the defective emissive element. Step 806 gcleans the transfer head to remove the defective emissive element, andStep 806 h recoats the transfer head with a liquid phase polymer.Alternatively, Step 806 i discards the transfer head with the attacheddefect item.

In the case of the adhesive mechanism, Step 806 j provides apick-and-remove deformable contact surface area transfer head that isadhesive with respect to a defective emissive element. Step 806 kexpands the transfer head deformable contact surface area to contact adefective emissive element, and in response to the contact, Step 806 lattaches the defective emissive element to the transfer head. Moreexplicitly, in Step 606 j the deformable contact surface may initiallybe a first flat surface area, and Step 806 k expands the transfer headdeformable contact surface area to create a second convex surface areato contact a defective emissive element positioned in a substrate well.Step 806 m discards the defect item.

In one aspect where the emissive substrate comprises an array of wellspopulated with emissive elements, Step 808 populates empty wells withreplacement emissive elements using a repurposed robotic pick-and-removeprocess as follows. Step 808 a attaches a replacement emissive elementto a pick-and-remove transfer head. Step 808 b positions the replacementemissive element on the emissive substrate top surface at a locationproximate to a well to be populated. Step 808 c translates thereplacement emissive element across top surface. In response totranslating the replacement emissive element over an opening in thewell, Step 808 d uses an elastic deformation force to direct thereplacement emissive element into the well.

A system and method have been provided for emissive substrate repair.Examples of particular process steps and hardware units have beenpresented to illustrate the invention. However, the invention is notlimited to merely these examples. Other variations and embodiments ofthe invention will occur to those skilled in the art.

We claim:
 1. A pick-and-remove method for repairing an emissive display,the method comprising: providing an emissive substrate including anarray of positioned emissive elements; inspecting the emissive substrateto determine defective array sites; and, in response to the inspection,using a pick-and-remove system to remove defect items from the defectivearray sites.
 2. The method of claim 1 wherein providing the emissivesubstrate includes providing an emissive substrate with an array ofwells, with emissive elements located in the wells, but not electricallyconnected to the emissive substrate.
 3. The method of claim 1 whereininspecting the emissive substrate to determine defective array sitesincludes determining defect items selected from a group consisting ofmissing emissive elements, misaligned emissive elements, mis-locatedemissive elements, non-functional emissive elements, and debris.
 4. Themethod of claim 3 wherein using the pick-and-remove system includesusing the pick-and-remove system to remove misaligned emissive elements,mis-located emissive elements, non-functional emissive elements, anddebris from a top surface of the emissive substrate.
 5. The method ofclaim 1 wherein using the pick-and-remove system includes using aphase-change holding mechanism.
 6. The method of claim 5 wherein usingthe phase-change holding mechanism includes: coating a pick-and-removetransfer head with a liquid phase polymer; subsequent to contacting adefect item with the transfer head, permitting the transfer head tocool; converting the polymer to a solid phase attached to the defectitem.
 7. The method of claim 6 wherein coating the pick-and-removetransfer head includes coating one pick-and-remove transfer head from aplurality pick-and-remove transfer heads; and, the method furthercomprising: subsequent to attaching the defect item, disposing of thetransfer head.
 8. The method of claim 6 further comprising: subsequentto attaching the defect item, cleaning the transfer head to remove thedefective emissive element; and, recoating the transfer head with aliquid phase polymer.
 9. The method of claim 1 further comprising:subsequent to removing the defect item, using a process selected fromthe group consisting of fluidic assembly or a repurposed pick-and-removesystem to populate empty wells with replacement emissive elements. 10.The method of claim 9 further comprising: subsequent to populating theempty wells with replacement emissive elements, reinspecting theemissive substrate to determine defective array sites; subsequent topassing reinspection, annealing the emissive substrate; and, in responseto the annealing, electrically connecting the emissive elements to theemissive substrate.
 11. The method of claim 2 wherein providing theemissive substrate includes providing light emitting diode (LED)emissive elements; and, wherein inspecting the emissive substrateincludes: irradiating the emissive substrate with ultraviolet (UV)illumination; photoexciting the array of LED; and, measuring LEDillumination at predetermined wavelengths to determine defective arraysites.
 12. The method of claim 1 wherein using the pick-and-removesystem to remove defect items using the electrostatic holding mechanismas follows: creating an electrostatic charge between a pick-and-removetransfer head and a defect item; attracting the defect item to thetransfer head in response to the electrostatic charge; and removing theelectrostatic charge to release the defect item emissive element fromthe transfer head.
 13. The method of claim 1 wherein using apick-and-remove system to remove defect items includes using an adhesiveholding mechanism as follows: providing a pick-and-remove deformablecontact surface area transfer head that is adhesive with respect to adefect item; expanding the transfer head deformable contact surface areato contact the defect item; and, in response to the contact, attachingthe defect item to the transfer head.
 14. The method of claim 9 whereinproviding the emissive substrate includes providing an emissivesubstrate with an array of wells populated with emissive elements;wherein populating empty wells with replacement emissive elements usingthe repurposed pick-and-remove system includes; attaching a replacementemissive element to a pick-and-remove transfer head; positioning thereplacement emissive element on the emissive substrate top surface at alocation proximate to a well to be populated; translating thereplacement emissive element across top surface; and, in response totranslating the replacement emissive element over an opening in thewell, using elastic deformation force to direct the replacement emissiveelement into the well.
 15. An emissive display repair system, the systemcomprising: an inspection subsystem for inspecting an emissive substratewith an array of wells, with emissive elements located in the wells, butnot electrically connected to the emissive substrate, and determiningdefective array sites; and, a pick-and-remove subsystem for removingdefect items from the emissive substrate.
 16. The system of claim 15wherein the emissive elements are light emitting diodes (LEDs); whereininspection subsystem comprises: an illuminator for irradiating theemissive substrate with ultraviolet (UV) spectrum light andphotoexciting the LEDs; and, a dual-mode image sensor for measuringvisual contrast and edge detection in a first mode, andwavelength-specific filtration in a second mode to identify functionalLEDs.
 17. The system of claim 15 wherein the inspection subsystemdetermines defect items selected from a group consisting of missingemissive elements, misaligned emissive elements, mis-located emissiveelements, non-functional emissive elements, and debris.
 18. The systemof claim 17 wherein the inspection subsystem determines a defect itemselected from the group consisting of misaligned emissive elements,mis-located emissive elements, non-functional emissive elements, ordebris; and, wherein the pick-and-remove subsystem uses a roboticpick-and-remove device to remove the defect item.
 19. The system ofclaim 18 wherein the robotic pick-and-remove devices uses a holdingmechanism selected from the group consisting of electrostatic,mechanical, and adhesive.
 20. The system of claim 19 wherein theelectrostatic pick-and-remove device includes a transfer head capable ofcreating an electrostatic charge to attract the defect item to thetransfer head, and dissipating the electrostatic charge on the transferhead to release the defect item.
 21. The system of claim 19 wherein themechanical pick-and-remove device includes: a thermal transfer head; aliquid phase polymer coating overlying the thermal transfer head; and,wherein the thermal transfer head, subsequent to contacting a defectitem, is cooled to convert the liquid phase polymer to a solid phaseattached to the defect item.
 22. The system of claim 21 wherein thethermal transfer head is cleaned of the polymer to remove the defectitem, and recoated with a liquid phase polymer for subsequent use. 23.The system of claim 21 wherein the thermal transfer head is disposed ofwith the attached defect item.
 24. The system of claim 19 wherein theadhesive pick-and-remove device includes: a transfer head with adeformable contact surface area that is adhesive with respect to adefect item; and, wherein the transfer head becomes adhesively attacheda defect item in response to expanding the contact surface area size.25. The system of claim 24 further comprising: a replacement subsystemfor populating empty wells with replacement emissive elements, thereplacement subsystem using a process selected from the group consistingof fluidic assembly or a repurposed pick-and-remove device.